1. Field of the Invention
The present invention relates to a write and read objective lens for encoding and recovery of information upon a storage member and, more particularly, to a write and read objective lens for the encoding of high-density information upon a storage member, and further, a write and read objective for the recovery of high-density information which is optically encoded on a storage member.
2. Description of the Prior Art
For a write and read objective lens for encoding and recovery of information upon a storage member, it is necessary that the write and read objective for encoding and recovery of information on a storage member be utilizable with a plurality of wavelengths of light. This is required due to the use of the write and read objective lens as both the encoding objective lens and further as a reproducing lens for the recovery of the encoded information, the specific requirements for encoding and recovery differ greatly. A write objective lens must be of a high numerical aperture to provide a minimum spot size at the object, this provides for the highest signal to noise ratio of the recorded information and further provides a higher storage density of information upon the storage member.
To provide the maximum available power through the objective lens for use in encoding information, the objective lens must be as transparent as possible at the wavelength of light provided for the encoding of information (.LAMBDA.=4420 nm). This can be accomplished by providing a multi-layer, anti-reflection coating on the lens surfaces. For an objective lens to be used in reproducing systems of encoded information, it is required to guarantee a resolving power of at least 1.mu. due to the fact that the objective lens must recover the very small signals recorded at a high density. Moreover, the information recovered from the disc, which rotates at high speed, provides information for directing the objective lens to follow the recorded track, information for automatic focusing in addition to the stored video information. To insure that the objective lens recovers those kinds of information correctly, the flatness of field focussed by the objective lens should be high. To prevent the objective lens from contacting the storage member which would result in the destruction of the storage member and the objective lens, the working distance of the objective lens should be long.
Further, to accomplish automatic focussing, the objective lens should be compact and light in weight. As the light source provided for generation of light provided to the objective lens for recovery of information is generally of the monochromatic type (.LAMBDA.=6328 nm), it is effective for increasing the signal to noise ratio of the recovered signal as amplified after recovery by a photo detector that the objective lens be as highly transparent to light of this wavelength as possible. Therefore, to increase the transparency, it is necessary to provide a multi-layer, anti-reflection coating on the lens surfaces and to minimize the number of lenses constituting the objective lens.